1. Field of the Invention
The present invention relates to magnetic carriers used for charging toners contained in a two component-type electrophotographic developer comprising the carrier and the toner, the electrophotographic developers containing the same carrier, and to a developing method.
2. Description of the Related Art
One of the important properties required for a magnetic carrier used as one component of an electrophotographic developer is a quick charging property of a toner, which is another component of the electrographic developer. The magnetic carrier is required to charge the toner rapidly. In other words, the carrier is required to have a high charge build-up rate. If a portion of the toner supplied into the developing machine while printing does not charge rapidly, this portion of the toner will have too low an electrostatic charge for developing the image, and the toner may be scattered on printed sheets including on the background portion. As a result, the insufficiently charged toner will contaminate the prints, causing a phenomenon called xe2x80x9cphotographic fogginggingxe2x80x9d or xe2x80x9cchemical fogginggingxe2x80x9d. In order to eliminate the insufficient charging and the photographic foggingging, various countermeasures have been proposed conventionally. In particular, extensive studies have been carried out in order to obtain superior magnetic core materials in terms of properties of materials, particle size, the surface coating, and further research was carried out to find suitable resin materials and the effects of heat treatments on the properties of the resin coated carriers. In order to reduce the photographic fogging, several measures were conventionally proposed such as improving the charging property of the carrier for the toner and improving the electric resistance of the developer composed of the carrier and the toner.
Examples of conventionally proposed techniques follow. Japanese Patent Application, First Publication No. Sho 47-17435 proposes to employ a fluororesin. Japanese Patent Application, First Publication No. Sho 60-19156 discloses that coating of a silane-coupling agent makes it possible to improve image density to a satisfactory level and to decrease the photographic fogging of images. Japanese Patent Application, First Publication No. Sho 61-20464 proposes to incorporate a conductive material in a silicone resin.
Japanese Patent Application, First Publication No. Sho 64-91144 discloses that a preferable image quality having no photographic fogging, no uneven density, and no blurring can be obtained by application of a mechanochemical treatment to silicone resin coated carrier particles, wherein the mechanochemical treatment is carried out by stirring the coated carrier particles mechanically until the carrier particles can charge the toner approximately 1.2 to 2.5 times. Japanese Examined Patent Application, Second Publication No. Hei 6-73030 discloses that carrier particles coated with a coating layer formed by a fluororesin, in which fluororesin particles are dispersed, shows a high image quality having no photographic fogging and a high stability over time in maintaining the charge. Japanese Patent Application, First Publication No. Hei 11-125934 discloses that a sufficient amount of charge can be obtained for the toner from the beginning of the printing by applying an impact force on the carrier particles coated with a silicone resin so as to accumulate a 1.5 times larger amount of electrostatic charge in the developer, which results in obtaining a high quality image having no photographic fogging over a long term.
However, photographic fogging is a complicated phenomenon and a complete solution to this problem is not yet obtained. The carrier is required to have characteristics such that the carrier not only charges the toner quickly to a predetermined level, but also the predetermined level of electrostatic charge must be maintained constant while printing. This is because the image density must be maintained constant while printing. A technique compatible with the quick charging and the long-term charge maintenance property is still a problem to be solved, because the quick charging is related to preventing the photographic fogging and the long-term charge maintenance property is related to the time-dependent reliability of the image density.
Recently, the charging or electrification mechanism between the carrier and the toner has been extensively studied. It is assumed that one possible mechanism is that the toner is charged as a result of an electron exchange process between the toner and the carrier due to the difference of surface energy levels of the toner and the carrier when the carrier is in contact with the toner. The surface energy level is related to a property which is called a work function. In general, when two substances having different work functions are in contact, the substance having a lower work function has a tendency to donate electrons (i.e., is likely to be positively electrified).
Various methods are proposed for measuring the work function of a substance, and, in general, the work function is obtained by measuring photoelectrons emitted from an object when light beams of various wavelengths are irradiated onto the object. Since the photoelectrons will not be emitted if the irradiated light does not exceed a certain energy level, an observation of the relationship between the light wavelength (photon energy) and the photoelectron emission makes it possible to obtain the minimum energy at which the photoelectron emission starts. Not only light beams but also X-rays or electron beams may be employed as the radiation, and the measurement is performed in a vacuum chamber in order to carry out an accurate observation of photoelectron emission.
Recently, however, a low energy electron counting apparatus has been developed which carries out the measurement of the work function by irradiation of ultraviolet light on an object in air. The above-described apparatus is becoming widely used in the fields of developers including toners and carriers for measuring their work functions. Japanese Patent Application, First Publication No.9-10610 defines the relationship between work functions of the toner and the carrier. Japanese Patent (Granted) Publication No. 2954786 discloses that a toner having superior color reproducibility can be obtained by identifying the differences of work functions of three primary color toners.
However, merely the work function is not sufficient for clarifying the electrostatic charging behaviors of the carrier and the toner, although the work function is one of the measures with respect to the charging performance. In the conventional carriers and toners developed employing the work function as a measure of electrification behaviors, a superior carrier is not yet available, which has performances of both quick charging and the charge maintenance properties, both of which are respectively required for eliminating the photographic fogging of an image and for maintaining the image density.
An object of the present invention is to provide a method of developing electrostatic images by using a carrier for electrophotography, which is compatible not only for the quickly charging the toner but also for maintaining the necessary charge during printing.
The first aspect of the present invention provides an electrophotographic carrier by coating magnetic particles with a resin, wherein the carrier satisfies the following equation (B) greater than (xe2x88x9219.4)xc3x97(A)+31, wherein, (A) represents the carbon content in weight percent, and (B) represents the ratio of a square root ((CPS)xc2xd) of a number of emitted photoelectrons (CPS) to a excitation energy (eV).
The second aspect of the present invention is related to an electrophotographic developer, provided with a toner comprising at least a colorant and a binder resin and the above-described carrier.
The third aspect of the present invention provide a method for developing electrostatic images using the above described developer for the electrophotography.
According to the present invention, the use of the electrophotographic carrier that satisfies the above-described equation makes it possible to improve the charge build-up of the toner and to reduce the time until the toner is charged to a necessary amount of electrostatic charge. As a result, the chemical fogging can be eliminated and the image density can be maintained constant.
The inventors of the present invention prepared a number of binary developers composed of various carriers and toners and the charging and printing behaviors of toners were evaluated in order to examine the correlations between (A) corresponding to the carbon content (weight %) of the carrier, and (B) which is a ratio of the square root of the number of emitted photoelectrons ((CPS)xc2xd) and a excitation energy (eV). In test production of carriers, the type and the amount of resin for coating the core particles of the carrier were changed, and the shearing force applied to the carrier during production and the method of surface coating were changed. The relationship between (A) and (B) are shown in FIG. 3, by plotting (A) on the horizontal axis for (A) and plotting (B) on the vertical axis. The carriers which exhibit superior charging and printing behaviors are marked by ∘, carriers which exhibit relatively good charging and printing behaviors are marked by ¤, and carriers showing inferior charging and printing behaviors are marked by xc3x97. The inventors of the present invention discovered the fact that, as shown in FIG. 3, the carriers which have a superior charging and printing behavior have the value of (B) above the line expressed by (B)=[(xe2x88x9219.4)xc3x97(A)+31], that is, the value of (B) which is greater than the value obtaining by calculating the formula [(xe2x88x9219.4)xc3x97(A)+31].
The value of (A) is a carbon content of the carrier obtained by calculating from the amount of carbon dioxide and carbon monoxide generated by firing the carrier in oxygen flow.
Specifically, (A) is a value obtained by dividing the total carbon content obtained by measurement of amounts of carbon dioxide and carbon monoxide generated at the time of combustion of the resin coated carrier by the total weight of the carrier before combustion. Accordingly, the value (A) of the present invention means the carbon content (weight %) of the resin coated carrier.
The carbon content is obtained by the following conditions.
Sample weight: 0.5 g
Combustion temperature: 1250xc2x0 C.
Measuring time: 30 seconds
Measuring temperature: 25xc2x0 C.
Measuring humidity: 60%
The value (B) is obtained by the following conditions.
Test apparatus: AC-1 (Riken Keiki Co.)
Quantity of light: 500 nW
Anode voltage: 3300 to 3450 V
Distance between an object and a detector: 1 mm
Measuring range: 6.0 to 3.8 eV
Measuring time: 10 seconds /1 point
Measuring temperature: 25xc2x0 C.
Measuring humudity: 60%
Furthermore, explanations are provided below about the low energy electron spectroscopic apparatus and the value (B) obtained by the measuring values of the above apparatus. The low energy electron spectroscopic apparatus AC-1 produced by Riken Keiki Co. was used to measure the value (B) employed in the present invention. FIG. 1 shows a schematic constitution of AC-1. A 500 nW light source was used as the light source for the ultraviolet light. The light beam emitted from the light source is separated into optional beams having a wavelength in a range of 200 to 360 nm and the separated lights are used for irradiation of the sample surface. The light beams having a wavelength in a range of 200 to 360 nm have a light energy ranging from 6.0 to 3.4 eV. In the present invention, the monochromatic beam having an energy range from 6.0 to 3.8 eV is used as the light beam for irradiation in the present invention. When an object is swept by the monochromatic light beams in sequence from the lower energy beam to the higher energy beam, the detector starts detecting a photoelectron emission due to the photoelectric effect. The energy of the light beam at which the photoelectron emission starts is the value called a photoelectric work function (work function). Electrons emitted from the sample ionize oxygen molecules in air and the ionized oxygen molecules are transferred to the low energy electronic counter (detector), where the ionized oxygen molecules emit electrons, and the number of emitted electrons are counted. As a result, a linear relationship is obtained between the square root ((CPS)xc2xd) of the number of emitted electrons and the excitation energy (eV). The slope B in FIG. 2 is the value (B) of the present invention. That is, the slope B [(CPS)xc2xd/eV] is a ratio of the square root of the number of emitted electrons (CPS)xc2xd and the excitation energy (eV) and this slope B corresponds to the value (B) in the equation of the present invention.
Note that the number of emitted electrons is measured in CPS (Counts Per Second), which is the number of photoelectrons emitted from the sample surface per second. The excitation energy (eV) is the energy which is received by the sample.